The present invention is concerned with the grinder housing of a pressure chamber grinder, in which the grinder housing comprises a substantially cylindrical outer mantle, end walls, preferably two accelerating nozzles passing radially through the outer mantle, between which said nozzles there is an obtuse angle, as well as a discharge opening made into one of the end walls for the ground product.
In the pressure chamber grinder apparatuses known in prior art, the discharge ends of the accelerating nozzles extend into the interior of the grinding chamber, where the material-gas jets rushing out of the nozzles collide against each other at very high speeds so that the material particles in the jets are ground. In the main grinding chamber in the prior-art pressure chamber grinders, the material particles are, in principle, subjected to the grinding effect only once. In the case of materials that are difficult to grind, no satisfactory grinding result can be obtained with one passage through the grinder, but the discharge opening of the grinding chamber is directly connected to a classifier, whose discharge opening for the coarse fraction is directly connected to the grinding chamber, to the collision zone of the material-gas jets.
The solution mentioned above is not fully successful, for, in order that the best possible grinding result should be achieved, the solids content in the material-gas jets colliding against each other must be kept relatively high, whereas, in order that a satisfactory classification result should be obtained, a very large excess quantity of gas is required in the classifier. In order that the material particles should reach an ultrasonic speed in the accelerating nozzles, the solids content in the material-gas jets rushing through the nozzles must be kept relatively low. On the other hand, in principle, the working gas has accomplished its duty by the stage in which the material particles have been accelerated through the nozzles. It has, viz., been noticed that the working gas present in the grinding chamber has mainly a detrimental effect on the grinding process proper.
Moreover, in the grinder apparatuses known in prior art, it is not possible to control the solids contents in the material-gas jets between different stages, but a compromise must be made in the choice of the grinding conditions, which does not yield a fully satisfactory result.